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replace

Learn replacement with replace and lambda.

replace: swap matched parts

replace replaces every part that matches the pattern with a new string. The first argument is a Regex, the second is the replacement text.

val r = Regex("\\d+")
val result = "a1b22c".replace(r, "#")
// "a#b#c"

To swap digits with # you write the pattern once. replace scans every match in order and puts the new text in its place. Use replaceFirst to replace only the first match.

replace with a lambda: flexible transform

You can transform each match separately by passing a lambda. The lambda takes a MatchResult and returns the replacement string.

val r = Regex("\\w+")
val result = "hello world".replace(r) {
    it.value.replaceFirstChar { c -> c.uppercaseChar() }
}
// "Hello World"

To capitalise the first letter of each word, use replaceFirstChar. Pick the lambda form when you want a transform, not a fixed text. This lets you shape the whole string in one pass.

Practice Replace digits with #

With Regex("\\d+"), replace the digits in "a1b22c333" with "#"; print the result.

Starter code

Main.kt
fun main() {
    val r = Regex("\\d+")
    // Use replace to turn digits into diyez and print
}

Sample solution

Main.kt
val r = Regex("\\d+")
println("a1b22c333".replace(r, "#"))

Expected output

a#b#c

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Fill in the blank: replace digits

Question 
val r = Regex("\\d+")
val result = "a1b2".____(r, "#")
Answer 
val r = Regex("\\d+")
val result = "a1b2".replace(r, "#")

Explanation: replace swaps every match with the given text.

2 What does replace(r) { ... } do?

  • Replace with fixed text
  • Transform instead of fixed text Correct answer
  • Find the first match
  • Count all matches

Explanation: The lambda transforms each match into a different string.

3 Match method to outcome

Term Meaning
replace(r, "#") All with fixed
replace(r) { ... } Transform per match
replaceFirst(r, "#") Only the first
matches Does all fit

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together